1. Field of the Invention
This invention generally relates to liquid crystal display (LCD) semiconductor fabrication and, more particularly, to a system and method for fabricating a substrate with a plurality of areas with different qualities, using a laser annealing process, with a corresponding plurality of masks.
2. Description of the Related Art
Polycrystalline silicon (poly-Si) material is used as the active layer of poly-Si thin film transistors (TFTs), for the fabrication of active-matrix (AM) backplanes. Such backplanes can be used to fabricate AM liquid-crystal displays and can be also combined with other display technologies, for example organic light-emitting diodes (OLEDs).
Poly-Si material is typically formed by the crystallization of initially deposited amorphous Si (a-Si) films. This process can be accomplished via solid-phase-crystallization (SPC), i.e., by annealing the a-Si films in a furnace at appropriate temperature, for a sufficiently long time. Alternatively, laser annealing can also be used to achieve the phase transformation.
Conventionally, all crystallization techniques are applied to a given substrate in such a manner as to yield poly-Si film of a uniform quality throughout the substrate area. In other words, there is no spatial quality differentiation over the area of the substrate. The most important reason for this end result is the inability of the current methods to achieve such quality differentiation. For example, when a-Si film is annealed in a furnace or by rapid-thermal-annealing, all of the layer will be exposed to the same temperature, resulting in the same quality of poly-Si material. In the case of conventional laser annealing, some differentiation is possible, but the price, in terms of loss of throughput, is very high for the modest performance gains. Hence, even for conventional laser annealing, such quality differentiation is not practically feasible.
Recently, a new laser annealing technique has been developed that allows for significant flexibility in the manner that the technique is applied and the resulting film microstructure. This technique relies on lateral growth of Si grains using very narrow laser beams, which are generated by passing a laser beam through a beam-shaping mask and projecting the image of the mask to the film that is being annealed. The method is dubbed Laser-Induced Lateral Growth (LILaC).
FIG. 1 illustrates steps in an LC-ELA annealing process (prior art). As seen in Step 1, initially amorphous silicon film is irradiated by a laser beam that is shaped by an appropriate mask to an array of narrow “beamlets”. The shape of the beamlets can vary. In Steps 1-4, each beamlet is shaped as a straight slit of narrow width, approximately 3-5 microns (μm). This slit is represented in the figure as the two heavy lines. The width of the slit is the distance between these two lines. This width can vary, but ultimately it is dependent upon the attainable lateral growth length (LGL), which is defined as the distance crystals can grow laterally (inwardly) from the edges of the irradiated area. Typically, the beamlet width is designed to be slightly less than twice the corresponding LGL.
The sequence of steps in FIG. 1 illustrates the growth of long polysilicon grains by LC-ELA process. A step-and-repeat approach is used. The laser beamlet width (indicated by the 2 parallel, heavy black lines) irradiates the film and, then steps a distance (d), smaller than half of the lateral growth length (L), i.e. d<L/2. Using this step-and-repeat process, it is possible to continually grow crystal grains from the point of the initial irradiation, to the point where the irradiation steps cease. L is dependent upon a combination of film thickness and substrate temperature. For example, a typical value of L, for a 50 nanometer (nm)-thick film at room temperature, is approximately 1.2 microns (μm). The value of L is due to this slow beamlet advancement. At each step, grains are allowed to grow laterally from the crystal seeds of the polycrystalline silicon (poly-Si) material formed in the previous step.
FIG. 2 is a plan view of a conventional optical system mask (prior art). The above-described process is equivalent to laterally “pulling” the crystals, as in zone-melting-crystallization (ZMR) method or other similar processes. As a result, the crystal tends to attain very high quality along the “pulling” direction, in other words, the direction of the advancing beamlets (shown by the arrow in FIG. 1). This process occurs in a parallel fashion (from each slit on the mask) allowing for rapid crystallization of the area covered by the projection of the mask on the substrate. Once this area is crystallized, the substrate moves to a new (unannealed) location and the process is repeated.
FIG. 3 is a pictorial representation of a system using the above-mentioned optical projection and the step-and repeat-process (prior art). Due to the step-and-repeat aspect of the laser projection, as opposed to furnace processes that treat a substrate uniformly, the LILaC process has the potential of permitting intentional spatial variations in the quality of the poly-Si material that is formed. Such intentional variations can be beneficial for applications where multiple components are integrated on an LCD display, where each component has different specifications and material performance requirements.
It would be advantageous if different areas of a substrate could be formed to different quality standards, to suit the function of the substrate area.
It would be advantageous if an entire substrate did not have to be annealed to meet the more stringent quality requirements of one particular area of the substrate.
It would be advantageous if a step-and-repeat laser annealing process could be used to anneal different areas of a substrate to different quality levels, according to need.